1. Field of the Invention
This invention relates to improvements in the handling of semiconductor wafers and, more particularly, to an apparatus and a method for shifting the wafers in a first wafer carrier into a second wafer carrier in a one-step operation.
2. Description of Prior Art
During wafer processing, it is often necessary to transfer wafers from one wafer carrier to another. It is imperative that the wafers be handled in such a manner that (1) contamination is not introduced onto the wafers' surface and (2) the wafers are not damaged. Wafer carriers are typically used in supporting wafers in stacks. The carriers are open frameworks that hold the wafers in close proximity to one another and evenly spaced apart. Because the carriers are open, they allow the wafers to be exposed to contamination. In addition, when wafers are transferred between carriers, the wafers may be damaged.
Two basic methods are now in use for the bulk transfer of wafers between carriers, namely, the dump transfer and the mechanically assisted transfer.
The dump transfer, also known as flip transfer, involves aligning a full carrier held open-side-up, to an empty carrier held open-side-down. The two carriers are kept in alignment while their relative positions are exchanged. Gravity causes the wafers to slide or fall from the first carrier into the other carrier.
Among the disadvantages of the dump transfer method are:
1. Wafer damage--During the transfer the upper surfaces of the wafers rub against the grooves of the carriers. Also, the movement of the wafers is hard to control, and wafers are often stopped abruptly by the bottom of the carrier into which they are transferred. Either situation can damage the wafers or their brittle photoresist coatings and cause contamination of the wafers by particles or shards.
2. Contamination due to handling--The nature of the transfer requires that an operator's hand be placed above the wafers during the operation. Contamination of the wafers can be caused by dust falling or being blown from the hand or glove onto the wafers.
3. Difficulty of operation--Dump transfer requires the operator's full attention, both hands, manual dexterity and good hand-eye coordination.
4. Misalignment--Registry pins and holes are used on standard carriers as alignment aids. The registry pins on either carrier may be bent or broken off, or the carriers warped, causing misalignment of the carriers during the transfer. Misalignment can cause wafers to be damaged or dropped during the transfer.
5. Contamination due to operator's breath--The need to align the carriers carefully often causes the operators to hold the carriers near their faces. This can lead to contamination of the wafers by residues in the operators' breath, for example, tobacco smoke.
6. Inflexible wafer orientation--Registry pins and holes are used on standard carriers to allow the carriers to be aligned in one relative position only. Correct alignment insures that the wafers will remain in the same orientation in all carriers into which they are transferred. However, correct alignment of carriers does not insure correct orientation of the wafers within the carriers. If wafer orientation needs to be changed, the registry pins make it difficult to do so.
The mechanically assisted transfer involves the use of a rotating device that transfers the wafers onto a soft stop and lowers them into a waiting carrier, or a moving arm that pushes the wafers from one carrier into another. Some units are power assisted, while others are manually operated.
Among the disadvantages of the mechanically assisted transfer are:
1. Contamination due to moving parts--Moving parts in contact with each other generate particulates. In general, moving parts require lubrication, which is itself a potential contaminant.
2. Cleanability--The units are difficult to clean because of their complicated construction. Often their components are sensitive to the standard cleaning agents used in the process area.
3. Maintenance--Moving parts require regular maintenance and/or adjustment.
4. Size--The units take up a large amount of needed table space.
5. Cost--The high cost of the units usually means that just a few are purchased and located in a central area, for use on "critical" transfers. Carrying wafers to and from a central location is inconvenient and likely to cause contamination. The inconvenience can also lead to the operators using the dump transfer instead.
6. Portability--Many units are not portable, being heavy or cumbersome. Lack of portability limits their usefulness and serviceability. Because of the drawbacks associated with conventional wafer transfer techniques, a need has arisen for an improved apparatus and method for transferring wafers from a first wafer carrier to a second wafer carrier in a controlled one step process that avoids wafer contamination and damage, is cost and space effective, and does not require special skills.